The present disclosure is directed to a method of analysis of waveforms obtained in acoustic logging to extract information about the formation which is not obtained in standard presentation of the waveform. One purpose is to obtain formation fracture information. It is intended to be used in data processing and analysis of sonic waveforms obtained from the use of a full waveform acoustic logging device. The acoustic logging device incorporates one or more transmitters which sends an acoustic wave into the adjacent formations, and which also includes one or more receivers which provide a recording of the received signal back from the formation. The acoustic waveform presentation typically includes a complex waveform, which when inspected, normally provides graphic evidence of the compressional component of the waveform, the subsequently occurring shear component of the waveform and the last component which arrives, namely, the Stoneley component. These are sometimes symbolically represented as the P, S and S.sub.t components. The acoustic signals from a particular receiver are recorded as a function of depth in the well borehole, and are presented by recording for a time interval sufficient to record all phases described above. The data is normally presented with a horizontal scale measured in microseconds, typically extending out to about 4,000 microseconds. The recorded waveform signals are typically presented on strip chart paper, film or the like and provides a graphic representation which includes refracted waves. Sometimes, reflected events called oblique events can be recognized in the recorded data. The data is analyzed with well known procedures to obtain an interpretation. In the event oblique events cannot be sufficiently recognized and evaluated, the present procedure enhances the presentation of the recorded data so that oblique events can be recognized and evaluated.
The improved data processing and interpretation approach taught by the present disclosure is particularly useful in locating oblique events and especially oblique fractures. One ordinarily envisions the formations encountered by a well borehole as a stack of horizontal layers which layers may have different sonic transmission characteristics and which have interfaces between layers. Such a representation may well prevail as the norm, but geological events cause variations from the horizontal layer model. Assume for purposes of example that the particular region which is being drilled previously experienced stress from an upthrust or other geological event. Such events will typically create fractures in the various formations. Knowledge of the location of such fractures including their angle is extremely helpful to determining information about the formation; it is also helpful to measure the location and dip angle of such fractures. In conjunction with other data, the data provided by the present disclosure is useful in interpretation of a sonic log and in therefore useful to provide further information after log interpretation. In fact, the information provided is enhanced so that fractures which would otherwise be obscure and missed can then be located.
The analysis and procedure set forth in the present disclosure involves the recording and presentation of sonic waveforms from a sonic logging operation conducted in a conventional fashion. Such data is captured and presented in the ordinary strip chart. That might provide a first valuable insight into the formations. However, the present procedure contemplates further processing. After adjacent recorded waveforms are recorded, analysis is first conducted to locate the precise time of arrival of the compression wave. Time shifting of one waveform compared to the next is done so that the P wavefront arrival in both waves is shifted to a common time for the two waveforms, and the difference between the two adjacent waveforms in then recorded. In other words, the two adjacent waveforms are presented in a stacked fashion. This involves alignment of the P event, or the later arriving S events. Alternately, the Stoneley event can likewise be stacked, and the difference between adjacent waveforms then presented. The presentation of the time shifted and subtracted waveforms provides a completely new set of waveforms. This set suppresses refracted and guided modes and enhances the obliquely arriving wavefront. For this set of waveforms, time dependent characteristics are measured. Such measurements include the relative phase and the first derivative of the phase measurement which is signal frequency. The waveform magnitude is represented, preferably in the form of a normalized value dependent on the vector amplitude. This is the modulus of the envelope of the complex waveform. The stacked depth dependent waveforms are enhanced in these three characteristics, and when the three characteristics are represented utilizing known color coding display procedures and in conjunction with the time shifted and stacked presentation of traces, obliquely arriving reflective modes are presented in a more clear and identifiable fashion. One purpose of the present procedure is to process acoustic waveform data in a new and different fashion to enhance the oblique events caused by fractures and to suppress the regular refracted and guided waves. This processing procedure can be applied to data obtained from practically all types of formations, and is thus available for use with conventional traces obtained heretofore. In fact, it can be used with an old recording of waveforms. That is, newly obtained data as well as old data can be handled and presented in a fashion which enables oblique event identification.